If you consider willpower to be a mystical trait shared by a lucky few -- or a hidden reserve that can be accessed only in dire straits -- you're not alone. Most people don't think they have much self-discipline. When two million online participants were asked by noted positive-psychology researcher Martin Seligman to choose their greatest strengths out of a list of 24 traits, self-control ranked at the very bottom. But the latest science shows that we all have willpower, says Roy Baumeister, a professor of psychology at Florida State University who cites the survey in "Willpower: Rediscovering the Greatest Human Strength," which he coauthored. "We should give ourselves more credit," Baumeister says.

Perhaps because willpower is viewed as an elusive, heroic virtue, we don't realize that we actually use it every day. "We're used to thinking of willpower in terms of our failures only," says Kelly McGonigal, a psychologist at Stanford University and author of the new book "The Willpower Instinct: How Self-Control Works, Why It Matters, and What You Can Do to Get More of It." But we're exercising our determination all the time, when we don't blow up at an employee who has made a clueless mistake, for instance, or when we pass up a cocktail and assume the responsibility of being the designated driver.

There's more good news: Our ability to access willpower is very much within our control. It's also in our best interest. Psychologists have found that self-control and intelligence are the two traits that predict -- in the parlance of the field -- "positive outcomes" in life. Having strong self-control means being less likely to make bad decisions that could negatively impact health (like having that fourth cookie) or the future (racking up a credit card bill). Willpower -- a complex mix of genetics, personality, goals, values, even a person's energy levels -- makes it possible to write a report in advance of the due date, hold off on buying those shoes, avoid arguing with a spouse, and get off the couch and into a Zumba class. Willpower, in other words, is on our side. It's looking out for our long-term goals -- and it's managing stress by averting problems.

"Since willpower is the ability to do what you really want, it's self-evident that the more you're able to use it, the happier you'll be," McGonigal says. "The problem is that people see it as forcing yourself to do what you don't want to do." A more helpful and constructive perspective, perhaps: "Defining willpower as having the strength or willingness to do what matters most to you," McGonigal says.

Pigeonholing ourselves as inherently weak-willed, she believes, lets us off the hook too easily. Why are scientists more confident in our stores of self-control than we are? Recent research provides some intriguing clues.

Willpower Fact: We're Wired for It

We may think of willpower as strictly a mental faculty, but it has a physiological basis, McGonigal says. It's rooted in an instinct, just like its better-known cousin, the "fight-or-flight" response. We're all aware of that one: Faced with an external stressor, whether it's a serious threat (a mugger on a darkened street) or an irksome frustration (waiting in an endless line), our heart rate and blood pressure increase and our breathing quickens.

Probably less familiar: "pause and plan," which is the instinct for self-control. "When your brain detects an impulse that threatens your immediate well-being or long-term goals, your heart rate and nervous system slow down," she explains. "Energy from the body is hurried to the brain to calm down an emotion or inhibit a craving."

Why don't we recognize "pause and plan" as easily as we do "fight or flight"? Because the stress response is related to immediate survival, it kicks in faster, and for the smallest of threats, McGonigal says. "Pause and plan," on the other hand, doesn't feel as obvious; it's trickier to notice. "Even though it's an instinct, it's harder for the brain to do, because it requires more energy," she says.

Willpower Fact: It Fluctuates

Yes, it's true: We have a limited amount of willpower, and it's important for us to use it effectively. When we use self-control (an imperceptible brain shift), we're actually drawing on, biologically speaking, glucose -- the sugar derived from the nutrients we consume. It's the same energy that powers muscles and organs. "The neurotransmitters that enable the brain to function are made from glucose," Baumeister explains. "Self-control is a complicated psychological event. It draws on your body's energy supply, which is limited."

Baumeister's research has shown that low levels of glucose correlate to poor performance on self-control tests, which makes sense: We all know that we're not at our decision-making best when we're famished. Every decision that's not habitual -- choosing from 20 bottles of shampoo at the store -- uses willpower, McGonigal says. "It taxes the system at a very low level." However subtle, that taxing effort takes its toll.

This may help explain another important finding: Our willpower levels inevitably fluctuate. Everybody has moments of strength and moments of weakness. "No one's self-control is perfect all the time," Baumeister says. So when there's a lapse, it makes sense to reframe the situation, thinking, "Maybe I was a little depleted, and I should manage myself better in the future," he says -- not, "I lack strength of character."

Willpower, in many ways, is like a muscle: It gets tired with use. "If you run for 10 miles and do a spin class, your quadriceps will need to recover," McGonigal says. "The willpower muscle works in a similar way. Once you use your strength, you need to recover and refuel." Just as someone would improve at the gym by progressively doing more demanding drills, a person can strengthen self-control by slowly and progressively challenging it, building up the brain's ability to tap into willpower.

Willpower Fact: Guilt Doesn't Work

To be sure, willpower is connected to success, and people with less self-control are more apt to make bad decisions. But labeling oneself as "bad" after eating an extra slice of pizza is unlikely to produce positive changes in behavior. In fact, it can lead to self-sabotage. Chastising creates guilt and shame, McGonigal says, emotions that trigger stress. So if you're primed to deal with anxiety in an unhealthy way, such as overeating, remorse over not exerting restraint will lead you right back to the thing you're trying to quit. "As soon as you introduce guilt, it shifts your body into a state where it makes you want whatever you feel ashamed about," McGonigal says. She cites a study in which people were asked to imagine how badly they'd feel after eating a piece of cake; this visualization triggered self-denigration, which paradoxically, made the participants more likely to eat the cake.

At the other end of the spectrum: feeling over-confident about good behavior. Patting yourself on the back for taking an extended turn on the treadmill is fine, as long as success isn't used as an excuse to revert to old behavior. Repeated studies have shown that we easily fall prey to what psychologists call "moral licensing," McGonigal says. If we think we've succeeded at something, we are likely to reward ourselves -- and often in a way that's in direct conflict with our goals. "If we say we're being good for doing our taxes or for eating a salad, then we give ourselves permission to sabotage our goals," she says. The result: We end up taking one step forward and one step back.

Willpower Fact: There Are Three Parts

It may sound monolithic, but willpower is a multifaceted thing: In fact, it comprises three separate powers. Using them in tandem can make it easier to create a new habit. There's "I will" power: the ability to do something we don't want to do because it's good for us (go to the gym at the crack of dawn before heading to the office). "I won't" power is the ability to stay away from things that are bad for us (playing an umpteenth round of Angry Birds before going to bed). And "I want" power -- making decisions based on goals ("Staying healthy, for my sake and my family's, means a lot to me.").

"People can be strong in one area, but not others -- like they have great self-restraint but are terrible procrastinators," McGonigal says. The secret to changing habits lies in paying attention to all three of the powers: "They all work together," McGonigal says. "I won't" and "I will" actions help create conditions that make change possible. "I want" power keeps the long-term reason for making a change front and center, prompting your willpower to enact itself. "Without the 'I want' power, it feels like gritting your teeth and bearing it, which is something very different," McGonigal says. "You're training to do something that's inconsistent with what you want."

Creating a new habit can take anywhere from an hour to a year, Baumeister and McGonigal say. But once new behavior is solidly ingrained -- through repetition or because someone begins to view the change as consistent with her identity (say, being a vegetarian) -- willpower is no longer necessary. At that point, the willpower reserve is primed for the next challenge -- and it's ready to be accessed for something else.